The present invention relates to quantum cascade lasers. More particularly, it relates to lasers comprising two electrodes for applying an electric control filed and a waveguide placed between the two electrodes and which comprises:
a gain region consisting of several layers which each comprise alternating strata of a first type each defining a quantum barrier and strata of a second type each defining a quantum well, these strata consisting of first and second semiconductor materials, respectively forming barriers and wells, and
two optical confinement layers placed on each side of the gain region.
In the present document, the term xe2x80x9cstratumxe2x80x9d refers to a thin layer having a degree of uniformity in its composition, and xe2x80x9clayerxe2x80x9d to a set of strata having one and the same function.
A laser of this type is described in patent U.S. Pat. No. 5,457,709. It consists of strata of first and second types, which respectively form barriers and quantum wells. The materials forming the barriers and wells are chosen so that they have a lattice parameter substantially equal to that of the substrate, so as to preserve the single crystal structure throughout the thickness of the laser.
The difference in crystal potential of the first and second materials respectively constituting the strata of the first and second types defines, by quantum effect, one or more two-dimensional states called subbands.
Each of the layers has an active region and an energy relaxation region. The application of an electric field to the terminals of the electrodes generates a current of charge carriers, especially inside the gain region.
The emission of laser radiation is generated by the transition of charge carriers in the active region from a first to a second subband. In general, these charge carriers are electrons. This phenomenon, called intersubband transition, is accompanied by the emission of a photon.
More specifically, the wells of the active zone comprise at least three subbands, respectively called upper, middle and lower subbands. Photons are emitted during a transition between the upper and middle subbands. This transition is made possible since the population of the middle subband is reduced by transfer of its electrons toward the lower subband, with emission of an optical phonon. For this to be possible, it is necessary that the energy lost by a charge carrier passing from the middle subband to the lower subband is larger than or equal to that of the optical phonons specific to the material used.
The main aim of the present invention is to improve this type of laser. To this end, each layer of the gain region has an injection barrier and an active region consisting of at least three pairs of strata, each pair consisting of a stratum of the first type and a stratum of the second type. The strata of the active region of each layer are arranged so that each of the wells has at least a first upper subband, a second middle subbands, and third and fourth lower subbands, the potential difference between the second and third subbands, on the one hand, and the third and fourth subbands, on the other hand, being such that the transition of an electron from the second to the third subbands, or from the third to the fourth, emits an energy said to be corresponding to that needed for the emission of an optical phonon of the single crystal in question.
Advantageously, the laser according to the invention comprises at least four strata of the second type.
In such lasers, the first and second materials, respectively forming barriers and wells, are deposited successively, while checking that the latter are pure or suitably doped, each stratum having a uniform composition, except for a few atomic layers adjoining the neighboring strata. By proceeding in this way, a succession of wells and barriers having almost vertical sides is obtained. Now, it has been noticed that the electrons tend to diffuse at the interfaces of these strata.
More specifically, the study carried out by K. L. Campman et al., and published under the title xe2x80x9cInterface roughness and alloy-disorder scattering contributions to intersubband transition linewidthsxe2x80x9d in Appl. Phys. Letters 69 (17), Oct. 21, 1996, has shown that the interfaces of the strata have a determining effect on the enlargement of the intersubband transition, which generates an increase in the threshold current.
In order to reduce this current, and in a particularly beneficial embodiment, the strata of the first and of the second type respectively have concentrations of 100% of the first and of the second materials in their middle parts, while between two middle parts, the strata consist of an alloy of the two materials, the concentration of which varies continuously.
Generally, the laser comprises a substrate, made of indium phosphide (InP), on which the various layers are placed.
With an InP substrate, it is advantageous for the second material to be InGaAs and the first to be chosen from AlGaAs, InP and AlInAs.
Depending on the applications, the wavelength of the laser radiation must be relatively short. A study carried out by J. Faist et al., entitled xe2x80x9cShort wavelength quantum cascade laser based on strained compensated InGaAs/AlInAsxe2x80x9d and published in Appl. Phys. Letters vol. 72, No. 6 of Feb. 9, 1998, has shown that it is possible to increase the difference in crystal potential of the two materials forming the strata and, consequently, to reduce the wavelength of the photons emitted. It is for this reason, advantageously, that the first and second materials are chosen such that they have lattice parameters one of which is greater, the other of which is smaller than those of the substrate.
In some applications, it is necessary to have radiation exhibiting a narrow emission spectrum. To this end, the confinement layer opposite the substrate has a structure defining a diffraction grating having a pitch equal to a multiple of a half wavelength in the crystal with the desired emission spectrum.